LICENSE: clarify

1) The GPL says that "if the Program does not specify a version number
of this License, you may choose any version ever published by the Free
Software Foundation".  This is not true, QEMU includes parts that are
v2-only.

2) Provide a default for files with no licensing information.

3) It is not just hardware emulation that is under BSD license.

4) Restrict GPLv2-only contributions to user mode emulation (due to
code from Linux) and PCI passthrough (due to code from Neocleus).

5) The rules were initially set by Fabrice but are being amended by
other people (already in commit ee12e1f3, LICENSE: There is no libqemu.a
anymore, 2011-11-15).  Do not put words in his mouth.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Andreas Färber <afaerber@suse.de>
Message-id: 1375251592-2537-3-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>